TradesHow this works
A footing spreads a load out until the pressure on the soil is one the soil can carry. Divide the load by the soil's bearing value and you have the bearing area you need. A pad or pier footing is sized square from that area; a wall footing is sized by width from the load on each foot of wall. This tool does that and applies the practical minimums for width and thickness.
The soil almost always governs, so be honest about what you have under the footing, and remember the two things width does not cover: the footing still has to reach below the frost line, and anything beyond light residential belongs with an engineer.
Common questions
How big does a footing need to be?
Big enough that the load spread over its bearing area does not exceed what the soil can carry. Divide the load by the soil bearing pressure to get the required area, then a pad is sized square from that area and a wall footing by width. Weak soil needs a wider footing for the same load.
What soil value should I use?
IRC gives presumptive values you can use without a soils test: 1500 psf for clays and silts, 2000 for sands, 3000 for gravels, and more for rock. But anything above 2000 psf requires an engineering evaluation to claim, and if you are not sure, assuming the weakest 1500 psf is the safe default.
Does this cover footing depth?
No. This sizes the width and thickness for bearing. The footing also has to sit below your local frost line and on undisturbed or compacted soil, which is a separate depth requirement set by your climate and code.
Sources: IRC Table R401.4.1 presumptive soil load-bearing values + R403.1 footing sizing from tributary load and allowable soil pressure; practical footing minimums (IRC). Engine version 1.0.0. Risk tier 2.